Chemistry of Igneous Rocks

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Chemistry of Igneous Rocks

• Characterization of different types (having
  different chemistries)
• Ultramafic ? Mafic ? Intermediate ? Felsic
• Composition commonly presented in weight of the
  oxides
• 40-78 SiO2
• 12-18 Al2O3

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Melts

• Liquid composed of predominantly silica and
  oxygen. Like water, other ions impart greater
  conductivity to the solution
• Si and O is polymerized in the liquid to
  differing degrees how rigid this network may
  be is uncertain
• Viscosity of the liquid ? increases with
  increased silica content, i.e. it has less
  resistance to flow with more SiO2 related to
  polymerization??
• There is H2O is magma ? 2-6 typically H2O
  decreases the overall melting T of a magma, what
  does that mean for mineral crystallization?

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• Minerals which form are thus a function of melt
  composition and how fast it cools
  (re-equilibration?) ? governed by the stability
  of those minerals and how quickly they may or may
  not react with the melt during crystallization

rock
Mg2
Fe2
cooling
Mg2
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Processes of chemical differentiation

• Partial Melting Melting of a different solid
  material into a hotter liquid
• Fractional Crystallization Separation of initial
  precipitates which selectively differentiate
  certain elements
• Equilibrium is KEY --? Hotter temperatures mean
  kinetics is fast

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Melting

• First bit to melt from a solid rock is generally
  more silica-rich
• At depth in the crust or mantle,
  melting/precipitation is a P-T process, governed
  by the Clausius-Clapeyron Equation Slope is a
  function of entropy and volume changes!
• But with water when minerals precipitate they
  typicaly do not pull in the water, melt left is
  diluted ? develop a negative P-T slope

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Melt-crystal equilibrium 1

• Magma at composition X (30 Ca, 70 Na) cools ?
  first crystal bytownite (73 Ca, 27 Na)
• This shifts the composition of the remaining melt
  such that it is more Na-rich (Y)
• What would be the next crystal to precipitate?
• Finally, the last bit would crystallize from Z

X
Y
Z
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Melt-crystal equilibrium 1b

• Precipitated crystals react with cooling liquid,
  eventually will re-equilibrate back, totally
  cooled magma xstals show same composition
• UNLESS it cools so quickly the xstal becomes
  zoned or the early precipitates are segregated
  and removed from contact with the bulk of the
  melt

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Why arent all feldspars zoned?

• Kinetics, segregation
• IF there is sufficient time, the crystals will
  re-equilibrate with the magma they are in and
  reflect the total Na-Ca content of the magma
• IF not, then different minerals of different
  composition will be present in zoned plagioclase
  or segregated from each other physically

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• What about minerals that do not coexist well do
  not form a solid solution are immiscible??

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• More than 1 crystal can precipitate from a melt
  different crystals, different stabilities
• 2 minerals that do not share equilibrium in a
  melt are immiscible (opposite of a solid
  solution)
• Liquidus ? Line describing equilibrium between
  melt and one mineral at equilibrium
• Solidus ? Line describing equilibrium with melt
  and solid
• Eutectic ? point of composition where melt and
  solid can coexist at equilibrium

Solidus
Diopside is a pyroxene Anorthite is a feldspar
Eutectic
Liquidus
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• Melt at composition X cools to point Y where
  anorthite (NOT diopside at all) crystallizes, the
  melt becomes more diopside rich to point C,
  precipitating more anorthite with the melt
  becoming more diopside-rich
• This continues and the melt continues to cool and
  shift composition until it reaches the eutectic
  when diopside can start forming

At eutectic, diopside AND anorhtite crystals
precipitate Lever Rule ? diopside/anorthite
(42/58) crystallize until last of melt
precipitates and the rock composition is Z
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• Melting ? when heated to eutectic, the rock would
  melt such that all the heat goes towards heat of
  fusion of diopside and anorthite, melts so that
  42 diopside / 58 anorthite
• When diopside gone, temperature can increase and
  rest of anorthite can melt (along liquidus)

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Thermodynamic definitions

• Gi(solid) Gi(melt)
• Ultimately the relationships between these is
  related to the entropy of fusion (DS0fus), which
  is the entropy change associated with the change
  in state from liquid to crystal
• These entropies are the basis for the order
  associated with Bowens reaction series ? greater
  bonding changes in networks, greater entropy
  change ? lower T equilibrium

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Melt-crystal equilibrium 2 - miscibility

• 2 component mixing and separation ? chicken soup
  analogy, cools and separates
• Fat and liquid can crystallize separately if
  cooled slowly
• Miscibility Gap no single phase is stable
• SOUP of X composition cooled in fridge Y vs
  freezer Z

100
SOUP
X
Temperature (ºC)
50
Y
0
fats
ice
Miscibility Gap
Z
-20
10
70
30
90
50
Water
Fat
fat in soup
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Melt-crystal equilibrium 2 - miscibility

• 2 component mixing and separation ? chicken soup
  analogy, cools and separates
• Fat and liquid can crystallize separately if
  cooled slowly
• Miscibility Gap no single mineral is stable in
  a composition range for x temperature

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Combining phase and composition diagrams for
mineral groups
Mica ternary
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SOLID SOLUTION

• Occurs when, in a crystalline solid, one element
  substitutes for another.
• For example, a garnet may have the composition
  (Mg1.7Fe0.9Mn0.2Ca0.2)Al2Si3O12.
• The garnet is a solid solution of the following
  end member components
• Pyrope - Mg3Al2Si3O12 Spessartine -
  Mn3Al2Si3O12
• Almandine - Fe3Al2Si3O12 and Grossular -
  Ca3Al2Si3O12.

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GOLDSCHMIDTS RULES

• 1. The ions of one element can extensively
  replace those of another in ionic crystals if
  their radii differ by less than approximately
  15.
• 2. Ions whose charges differ by one unit
  substitute readily for one another provided
  electrical neutrality of the crystal is
  maintained. If the charges differ by more than
  one unit, substitution is generally slight.
• 3. When two different ions can occupy a
  particular position in a crystal lattice, the ion
  with the higher ionic potential forms a stronger
  bond with the anions surrounding the site.

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RINGWOODS MODIFICATION OFGOLDSCHMIDTS RULES

• 4. Substitutions may be limited, even when the
  size and charge criteria are satisfied, when the
  competing ions have different electronegativities
  and form bonds of different ionic character.
• This rule was proposed in 1955 to explain
  discrepancies with respect to the first three
  Goldschmidt rules.
• For example, Na and Cu have the same radius and
  charge, but do not substitute for one another.

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COUPLED SUBSTITUTIONS

• Can Th4 substitute for Ce3 in monazite (CePO4)?
  
• Rule 1 When CN 9, rTh4 1.17 Å, rCe3
  1.23Å. OK
• Rule 2 Only 1 charge unit difference. OK
• Rule 3 Ionic potential (Th4) 4/1.17 3.42
  ionic potential (Ce3) 3/1.23 2.44, so Th4
  is preferred!
• Rule 4 ?Th 1.3 ?Ce 1.1. OK
• But we must have a coupled substitution to
  maintain neutrality
• Th4 Si4 ? Ce3 P5

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OTHER EXAMPLES OF COUPLED SUBSTITUTION

• Plagioclase NaAlSi3O8 - CaAl2Si2O8
• Na Si4 ? Ca2 Al3
• Gold and arsenic in pyrite (FeS2)
• Au As3 ? 2Fe2
• REE and Na in apatite (Ca5(PO4)3F)
• REE3 Na ? 2Ca2

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INCOMPATIBLE VS. COMPATIBLE TRACE ELEMENTS

• Incompatible elements Elements that are too
  large and/or too highly charged to fit easily
  into common rock-forming minerals that
  crystallize from melts. These elements become
  concentrated in melts.
• Large-ion lithophile elements (LILs)
  Incompatible owing to large size, e.g., Rb, Cs,
  Sr2, Ba2, (K).
• High-field strength elements (HFSEs)
  Incompatible owing to high charge, e.g., Zr4, Hf
  4, Ta4, Nb5, Th4, U4, Mo6, W6, etc.
• Compatible elements Elements that fit easily
  into rock-forming minerals, and may in fact be
  preferred, e.g., Cr, V, Ni, Co, Ti, etc.

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Changes in element concentration in the magma
during crystal fractionation of the Skaergaard
intrusion Divalent cations
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Changes in element concentration in the magma
during crystal fractionation of the Skaergaard
intrusion Trivalent cations
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THREE TYPES OF TRACE-ELEMENT SUBSTITUTION

• 1) CAMOUFLAGE
• 2) CAPTURE
• 3) ADMISSION

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CAMOUFLAGE

• Occurs when the minor element has the same charge
  and similar ionic radius as the major element
  (same ionic potential no preference.
• Zr4 (0.80 Å) Hf4 (0.79 Å)
• Hf usually does not form its own mineral it is
  camouflaged in zircon (ZrSiO4)

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CAPTURE

• Occurs when a minor element enters a crystal
  preferentially to the major element because it
  has a higher ionic potential than the major
  element.
• For example, K-feldspar captures Ba2 (1.44 Å
  Z/r 1.39) or Sr2 (1.21 Å Z/r 1.65) in place
  of K (1.46 Å, Z/r 0.68).
• Requires coupled substitution to balance charge
  K Si4 ? Sr2 (Ba2) Al3

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ADMISSION

• Involves entry of a foreign ion with an ionic
  potential less than that of the major ion.
• Example Rb (1.57 Å Z/r 0.637) for K (1.46 Å,
  Z/r 0.68) in K-feldspar.
• The major ion is preferred.

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Partition Coefficients

• How can we quantify the distribution of trace
  elements into minerals/rocks?
• Henrys Law describes equilibrium distribution of
  a component (we usedit for thinking about gases
  dissolved in water recently)
• aimin kiminXimin
• aimelt kimeltXimelt
• All simplifies to
• Often termed KD, values tabulated

http//www.earthref.org/databases/index.html?main.
htm
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Limitations of KD

• What factors affect how well any element gets
  into a particular rock???

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• However, most KD values reported close to
  equilibrium T-P values commonly encountered (?)
  and are reasonable, at least in terms of relative
  values between different elements

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Homework

• Chapter 8
• Problem 2, 6